Thermal energy can be directly converted into electric energy. Thermal energy converters are devices that directly convert thermal energy into electric energy. One type of such devices is a thermionic energy converter, in which heat is converted directly into electrical energy by thermionic emission. In this process, electrons are thermionically emitted from the surface of a metal by heating the metal and imparting sufficient energy to a portion of the electrons to overcome retarding forces at the surface of the metal in order to escape. Unlike most other conventional methods of generating electrical energy, thermionic conversion does not require either an intermediate form of energy or a working fluid, other than electrical charges, in order to change heat into electricity.
In its most elementary form, a conventional thermionic energy converter in an operational set-up consists of one electrode connected to a heat source, a second electrode connected to a heat sink and separated from the first electrode by an intervening space, leads connecting the electrodes to the electrical load, and an enclosure. The space in the enclosure is either highly evacuated or filled with a suitable rarefied vapor, such as cesium. Electrons are provided with enough thermal energy to be able to escape from one of the electrodes and travel to the other electrode. This is achieved in these devices by providing enough thermal energy to the electrons so that they have an energy that is at least the energy of the potential barrier to be overcome to travel from the emitting electrode to the collecting electrode.
The heat source supplies heat at a sufficiently high temperature to one electrode, the emitter, in a thermionic converter from which electrons are thermionically evaporated into the evacuated or rarefied-vapor-filled inter-electrode space. The electrons move through this space toward the other electrode, the collector, which is kept at a low temperature near the heat sink temperature. There the electrons condense and, upon closing the circuit, return to the hot electrode via external electrical leads and, for example, an electrical load connected between the emitter and the collector.
An embodiment of a conventional thermionic converter 100 is schematically illustrated in FIG. 1. These conventional devices typically comprise an emitter 110, or low electron-work-function cathode, a collector 112, or comparatively colder, high electron-work-function anode, an enclosure 114, suitable electrical conductors 116, and an external load 118. Emitter 110 is exposed to heat flow 120 which causes this cathode to emit electrons 122, thus closing the electric circuit and providing an electric intensity to load 118. As indicated above, inter-electrode space 130 in conventional thermionic converters is an evacuated medium or a rarefied-vapor-filled medium. The inter-electrode space in these converters is typically in the order of at least thousands of times greater than typical atomic dimensions. For example, the inter-electrode space in these converters can be in the order of about one tenth of a micrometer to one thousand or more micrometers, in which case the spacing between the electrodes is in the order of 103 Å to 107 Å.
The flow of electrons through the electrical load is sustained by the temperature difference between the electrodes. Thus, electrical work is delivered to the load.
Thermionic energy conversion is based on the concept that a low electron work function cathode in contact with a heat source will emit electrons. These electrons are absorbed by a cold, high work function anode, and they can flow back to the cathode through an external load where they perform useful work. Practical thermionic generators are limited by the work function of available metals or other materials that are used for the cathodes. Another important limitation is the space charge effect. The presence of electrons in the space between the cathode and anode will create an extra potential barrier which reduces the thermionic current. These limitations detrimentally affect the maximum current density, and thus present a major problem in developing large-scale thermionic converters.
Conventional thermionic converters are typically classified as vacuum converters or gas-filled converters. As indicated above, these converters have inter-electrode spacings that are at least several orders of magnitude greater than atomic dimensions. Vacuum converters have an evacuated medium between the electrodes. These converters have limited practical applications.
Attempts to reduce space-charge effects with vacuum converters have involved the reduction of the inter-electrode separation to the order of micrometers. Attempts to reduce the same effects with gas-filled converters have led to the introduction of positive ions into the cloud of electrons in front of the emitter. Nevertheless, these conventional devices still present shortcomings such as those related to limited maximum current densities and temperature regimes. A more satisfactory solution to converting thermal energy to electrical energy at lower temperature regimes with high efficiencies and high power densities has been sought with the use of solid state devices that still maintain an inter-electrode separation that is at least several orders of magnitude greater than atomic dimensions.
However, no converters have been provided that include inter-electrode separations in a range that is comparable with atomic dimensions. These converters are provided in the context of the present invention, with embodiments that have inter-electrode separations in the range of several tens of Ångstroms, a much reduced inter-electrode spacing. This reduction is typically in the range of several orders of magnitude with respect to inter-electrode separations in other converters that have inter-electrode separations in the range of not less than a few micrometers or a few tenths of a micrometer. The present invention provides such converters with inter-electrode separations that are comparable with atomic dimensions.
The reduction in inter-electrode spacing accomplished in the context of the present invention is not a mere change in the choice of design parameters or a matter of routine optimization of a length parameter. Miniaturization at the level provided by embodiments of the present invention requires the solution of design, manufacturing and modeling problems that are either not present in the context of conventional devices or that present themselves with different aspects and complexities. For example, microscopic electrode surface characteristics, such as surface irregularities, become relevant at such small inter-electrode separations, but such characteristics are slightly relevant, or even not relevant at all, in conventional devices with much larger inter-electrode separations. By way of another example, physical phenomena, such as tunneling, that are irrelevant in conventional devices, become very relevant when the inter-electrode separation is reduced to separations in the range of atomic length scales. Accordingly, special and complex characteristics of embodiments of the present invention such as those pointed out above by way of illustrative examples distinguish embodiments of the present invention from conventional devices that have much larger inter-electrode separations. As illustrated by the examples provided above, and described in the rest of the specification and drawings, these distinguishing features are not merely present in amount or extent, but in nature or kind of the devices themselves and of their design, manufacturing, and operational principles.